Above-ground carbon stocks, species diversity and fire dynamics in the Bateke Plateau

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Mitchard, Edward

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Ryan, Casey

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Nieto Quintano, Paula Adriana

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2019-08-07T12:10:20Z

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2019-08-07T12:10:20Z

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2019-07-03

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http://hdl.handle.net/1842/36002

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Savannas are heterogeneous systems characterised by a high spatial and temporal variation
in ecosystem structure. Savannas dominate the tropics, with important ecological functions,
and play a prominent role in the global carbon cycle, in particular responsible for much of its
inter-annual variability. They are shaped by resource availability, soil characteristics and
disturbance events, particularly fire. Understanding and predicting the demographic
structure and woody cover of savannas remains a challenge, as it is currently poorly
understood due to the complex interactions and processes that determine them. A predictive
understanding of savanna ecosystems is critical in the context of land use management and
global change.
Fire is an essential ecological disturbance in savannas, and forest-savanna mosaics
are maintained by fire-mediated positive feedbacks. Over half of the world’s savannas are
found in Africa, and over a quarter Africa’s surface burns every year, with fires occurring
principally in the savanna biome. These have strong environmental and social impacts. Most
fires in Africa are anthropogenic and occur during the late dry season, but their dynamics and
effects remain understudied.
The main objective of this research is to understand the floristic composition, carbon
storage, woody cover and fire regime of the mesic savannas of the Bateke Plateau. The
Bateke Plateau is savanna-forest mosaic ecosystem, situated mainly in the Republic of Congo,
with sandy Kalahari soils and enough precipitation for potential forest establishment (1600
mm/yr). Despite occupying 89,800 km2, its ecology and ecosystem functions are poorly
understood. This study combines two approaches: firstly experimental, setting up long term
field experiments where the fire regime is manipulated, and then observational, using
remote sensing to estimate the carbon storage and study the past history of the fire regime
in the region. I established four large (25 ha) plots at two savanna sites, measured their
carbon stocks, spatial structure and floristic composition, and applied different annual fire
treatments (early and late dry season burns). These treatments were applied annually during
3 years (2015, 2016 and 2017), and the plots were re-measured every year to estimate tree
demographic rates and the identification of the key processes that impact them, including
fire and competition. Field data were combined with satellite radar data from ALOS PALSAR,
and the fire products of the MODIS satellites, to estimate carbon stocks and fire regimes for
the entire Bateke Plateau. I also analyse the underlying biophysical and anthropogenic
processes that influence the patterns in Above-Ground Woody Biomass (AGWB) and their
spatial variability in the Bateke landscape.
The total plant carbon stocks (above-ground and below-ground) were low, averaging
only 6.5 ± 0.3 MgC/ha, with grass representing over half the biomass. Soil organic matter
dominate the ecosystem carbon stocks, with 16.7 ± 0.9 Mg/ha found in the top 20 cm alone.
We identified 49 plant species (4 trees, 13 shrubs, 4 sedges, 17 forbs and 11 grass species),
with a tree hyperdominance of Hymenocardia acida, and a richer herbaceous species
composition. These savannas showed evidence of tree clustering, and also indications of
tree-tree competition. Trees had low growth rates (averaging 1.21 mm/yr), and mortality was
relatively low (3.24 %/yr) across all plots. The experiment showed that late dry season fires
significantly reduced tree growth compared to early dry season fires, but also reduced stem
mortality rates. Results show that these mesic savannas had very low tree biomass, with tree
cover held far below its climate potential closed-canopy maximum, likely due to nutrient
poor sandy soils and frequent fires.
Results from the remote sensing analysis indicated that multiple explanatory
variables had a significant effect on AGWB in the Bateke Plateau. Overall, the frequency of
fire had the largest impact on AGWB (with higher fire frequency resulting in lower AGWB),
with sand content the next most important explanatory variable (with more sand reducing
AGWB). Fires in the Bateke are very frequent, and show high seasonality. The proportion of
fires that occurred in the late dry season, though smaller predictor, was also more important
than other factors (including soil carbon proportion, whether or not the savanna area was in
a protected area, annual rainfall, or distance to the nearest town, river or road), with a larger
proportion of late dry season fires associated with a small increase in AGWB. The results give
pointers for management of the savannas of the Bateke Plateau, as well as improving our
understanding of vegetation dynamics in this understudied ecosystem and help orient policy
and conservation.